1. Field of the Invention
This invention relates to a method for producing ring gears, especially ring gears of the hypoid, straight-bevel or spiral-bevel type for heavy-duty truck drive axles. In particular, the present invention relates to a method for producing ring gears including precision forging near net ring gear forgings which are finish ground on the tooth profiles after carburizing heat treatment to eliminate the need for lapped ring gear/pinion gear gear-sets.
2. Description of the Prior Art
Right angle drive trains for heavy-duty drive axles utilizing pinion gears/ring gear gear-sets are well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 3,265,173; 4,018,097; 4,046,210; 4,050,534 and 4,263,824, by reference to allowed U.S. patent application Ser. No. 761,262, filed Aug. 1, 1985, now U.S. Pat. No. 4,651,587, and assigned to the Assignee of this invention, and to SAE Paper No. 841085, the disclosures of all of which are hereby incorporated by reference. Such gear-sets are usually of the well known spiral-bevel or hypoid gear type or some modification or derivative thereof.
Forging processes for the production of gear forgings/gear blanks having at least partially formed teeth are well known in the prior art, especially for relatively smaller sized bevel gears, such as differential pinion and side gears, as may be seen by reference to U.S. Pat. Nos. 3,832,763; 4,050,283 and 4,590,782, the disclosures of which are all hereby incorporated by reference.
The ring rolling process whereby generally annular rings are ring rolled from ring rolling preforms is also well known in the prior art as may be seen by reference to U.S. Pat. Nos. 1,971,027; 1,991,486; 3,230,370; 3,382,693; and 4,084,419, and to Metals Handbook, 8th Edition, Volume 5, American Society for Metals, Pages 106 and 107, "Ring Rolling", the disclosures of all of which are hereby incorporated by reference.
In the past, due to the relatively massive size, ring gears for heavy-duty trucks have been produced by a method comprising the forging of a gear blank having outer diameter flash and a center slug, trimming of the forged gear blank, a normalizing heat treatment of the trimmed gear blank, extensive machining of the gear blank to rough and then final cut gear teeth therein, other machining of surfaces and mounting bores, a carburizing heat treatment, a lapping operation wherein the ring gear and a pinion gear are rotated in meshing engagement in a lapping compound, and then maintaining the ring gear and pinion gear as a matched set to be used only in connection with one another.
While the prior art method for producing ring gears for heavy-duty trucks has been utilized for many years as have the ring gears and ring gear/pinion gear-sets produced thereby, this method is not totally satisfactory as the billets used therein are of a considerably greater volume than the finished ring gear representing undesirably high material and heating costs, cutting of the gear teeth from the gear blanks is an expensive and time consuming operation and teeth formed by a cutting process do not possess the desirable grain flow characteristics inherent in gear teeth formed by a material deformation process and thus do not provide the performance of formed gear teeth. Also, as the lapped ring gear/pinion gear gear-sets are only usable as a matched pair, great care must be taken to maintain the gear-sets in matched pairs and damage to either the ring gear or pinion gear will render the entire gear set useless.
The forging of hollow members from rolled rings to save material is generally known in the prior art. However, this process usually is economical only for high volume production because ring rolling of the blanks requires a forming operation (on a forge press or hammer) to produce the annular preform to be ring rolled. The material savings, and other savings associated therewith, were not sufficient to make such a method economically desirable, especially as to the relatively larger more costly ring gears, in the volume and variety of sizes and ratios associated with heavy-duty drive axles (i.e. drive axles utilized with heavy-duty trucks, off-the-road construction vehicles and the like). This was because prior art production of preforms, as with most other forging operations, had the conventional wisdom that the preform die must be filled to nearly one hundred percent (100%) of its theoretical capacity and thus each different sized preform would require a separate die and, for relatively small lots, the material savings would be more than offset by the additional preform tooling and press setups normally required.